The present invention relates generally to an electronic timepiece, and more particularly, to an electronic wristwatch which stores power transferred by magnetic induction to a secondary power supply which in turn activates clock circuitry.
Extending the lifetime of batteries used in electronic wristwatches is a constant and major challenge. The size of batteries used in wristwatches are inherently limited by the size of the wristwatch especially in small-sized wristwatches.
Electronic wristwatches such as disclosed in U.S. Pat. No. 4,653,931, provide a relatively unlimited lifetime by using a solar battery. The solar battery is positioned on a display face such as on the dial of the wristwatch. A secondary battery or a charging capacitor is charged by the solar battery. A clock circuit is driven by the output of either the secondary battery or capacitor. Design of the wristwatch is limited since a black or blue solar battery is typically disposed on the dial. Therefore, electronic watches employing solar batteries have a limited appeal to purchasers who are attracted to a wristwatch based on its design.
Alternatively, an a.c. generator has been provided in the wristwatch with a clock circuit driven by the power generated therefrom. The a.c. electromotive force provided by the generator requires rectification. The most efficient rectifier circuit performs full-wave rectification by using a diode bridge which employs four diodes. Unfortunately, it is difficult to incorporate four diodes within the small space of a wristwatch. To ensure that the clock circuit operates continuously without error (even when the generator is in an inoperative state), the generated power must be stored in either a secondary battery or a capacitor (hereinafter referred to as the "secondary power supply") with the output therefrom used to continuously drive the clock circuit. The range of operating voltages of the clock circuit is limited. Therefore, the voltage of the secondary power supply must be maintained at a level which exceeds the lower limit of the operating voltage range of the clock circuit. The time required to charge the secondary power supply can be shortened by decreasing the storage capacity of the secondary power supply. A decrease in the storage capacity, however, accelerates the voltage lowering time (i.e. the time required to reach the lower limit) especially when the generator is in an inoperative state.
Accordingly, it is desirable to provide a rechargeable electronic wristwatch which includes an a.c. generator and which is not limited in terms of its design due to the type of secondary power source used. The clock circuitry of the electronic wristwatch should be operable over the entire voltage range of the secondary power supply. Additionally, the number of elements required for the rectifier circuit should be minimized.